In general, a liquid developer for electrophotography is prepared by dispersing an inorganic or organic pigment or dye such as carbon black, nigrosine, phthalocyanine blue, etc., a natural or synthetic resin such as an alkyd resin, an acrylic resin, rosine, synthetic rubber, etc., in a liquid having a high electric insulating property and a low dielectric constant, such as a petroleum aliphatic hydrocarbon, etc., and further adding a polarity-controlling agent such as a metal soap, lecithin, linseed oil, a higher fatty acid, a vinyl pyrrolidone-containing polymer, etc., to the resulting dispersion.
In such a developer, the resin is dispersed in the form of insoluble latex grains having a grain size of from several nm to several hundred nm. In a conventional liquid developer, however, a soluble dispersion-stabilizing resin added to the liquid developer and the polarity-controlling agent are insufficiently bonded to the insoluble latex grains, thereby the soluble dispersion-stabilizing resin and the polarity-controlling agent are in a state of easily dispersing in the liquid carrier. Accordingly, there is a fault that when the liquid developer is stored for a long period of time or repeatedly used, the dispersion-stabilizing resin is split off from the insoluble latex grains, thereby the latex grains are precipitated, aggregated, and accumulated to make the polarity thereof indistinct. Also, since the latex grains once aggregated or accumulated are reluctant to re-disperse, the latex grains remain everywhere in the developing machine attached thereto, which results in causing stains of images formed and malfunctions of the developing machine, such as clogging of a liquid feed pump, etc.
For overcoming such defects, a means of chemically bonding the soluble dispersion-stabilizing resin and the insoluble latex grains is disclosed in U.S. Pat. No. 3,990,980. However, the liquid developer disclosed therein is still insufficient although the dispersion stability of the grains to the spontaneous precipitation may be improved to some extent. Also, when the liquid developer is actually used in a developing apparatus, the toner adhered to parts of the developing apparatus solidified to form a film and the toner grains thus solidified are reluctant to re-disperse and are insufficient in re-dispersion stability for practical use, which causes the malfunction of the apparatus and staining of duplicated images.
In the method of producing resin grains described in aforesaid U.S. Pat. No. 3,990,980, there is a very severe restriction in the combination of a dispersion stabilizer to be used and monomer(s) being insolubilized for producing mono-dispersed latex grains having a narrow grain size distribution. Mostly, the resin grains produced by the aforesaid method are grains of a broad grain size distribution containing a large amount of coarse grains or poly-dispersed grains having two or more different mean grain sizes. In the aforesaid method, it is difficult to obtain mono-dispersed resin grains having a narrow grain size distribution and having a desired grain size, and the method often results in forming large grains having a mean grain size of 1 .mu.m or larger or very fine grains having a mean grain size of 0.1 .mu.m or smaller. Furthermore, there is also a problem that the dispersion stabilizer used must be prepared by an extremely complicated process requiring a long reaction time.
Furthermore, for overcoming the aforesaid defects, a method for improving the dispersibility, re-dispersibility and storage stability of resin grains by forming insoluble dispersed resin grains by copolymerizing a monomer being insolubilized with a monomer containing a long chain alkyl group or a monomer containing at least two polar groups as disclosed in JP-A-60-179751 and JP-A-62-151868 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Also, a method for improving the dispersibility, re-dispersibility and storage stability of resin grains by forming insoluble dispersed resin grains by copolymerizing a monomer being insolubilized with a monomer containing a long chain alkyl group or a monomer containing at least two polar groups in the presence of a polymer utilizing a di-functional monomer or a polymer utilizing a macromolecular reaction is disclosed in JP-A-60-185963, JP-A-61-63855, JP-A-62-166362 and JP-A-63-66567.
On the other hand, an attempt has recently been made to print a large number of prints such as more than 5,000 prints using a master plate for offset printing by electrophotography, and, as a result of improvement particularly in the master plate, it has become possible to print more than 10,000 prints of large size. Also, a noticiable progress has recently been made in shortening the operation time in an electrophotomechanical system and an improvement of quickening a development-fix steps in the system has been made.
Also, the rationalization of an electrophotomechanical system has been greatly required and practically, it has been attempted to prolong the maintenance time of a printing plate making machine. In this attempt, a liquid developer which can be used for a long period of time without being renewed has been required.
The dispersed resin grains produced by the methods disclosed in JP-A-60-179751, JP-A 62-151868, JP-A-62-166362 and JP-A-63-66567 yet show an unsatisfactory performance with respect to the dispersibility and re-dispersibility of the resin grains when the resin grains are used at a long interval of maintenance or the development speed is increased. Also, these resin grains show an unsatisfactory performance with respect to the dispersibility and re-dispersibility of the resin grains and the printing durability of plates obtained by the development with a liquid developer containing such resin grains.
In particular, there has been a problem in the improvement of re-dispersibility of the dispersed resin grains when the plate processing operation is improved by prolonging the interval of maintenance of the plate processing machine, or when the image quality of the reproduced image is improved in case of using a large size plate-making machine for a large size master plate (e.g., a size larger than A-3) without causing stains of the developing machine.